IJRIT International Journal of Research in Information Technology, Volume 2, Issue 9, September 2014, Pg. 528-531

International Journal of Research in Information Technology (IJRIT)

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ISSN 2001-5569

Dc Machines the Task of the Electro-Magnetic Conversions Ritika Gautam, Sahil Soni Department of electronics and communication engineerin, Dronacharyacollege of engineering, Khentawas, Farrukhnagar, Gurgaon-123506,India [email protected] , [email protected]

Abstract The steam age signalled the beginning of an industrial revolution. The advantages of machines and gadgets engaged in helping mass production and in improving the services spurred the industrial research. Thus a search for new sources of energy and novel gadgets received great attention. By the end of the 18th century the research on electric charges received a great boost with the invention of storage batterie’s. This enabled the research work on moving charges or currents. It was soon discovered that, these electric currents are also associated with magnetic field like a load stone. This led to the invention of an electromagnet. Hardly a year later the force exerted on a current carrying conductor placed in the magnetic field was invented. This can be termed as the birth of a motor. The evolution of these machines which went was very quick. They rapidly attained the physical configurations that are being used even today. system for use, with the availability of batteries for storage, d.c. generators for conversion of mechanical energy into electrical form and d.c. motors for getting mechanical outputs from electrical energy. The evolution of these machines was very quick. The d.c. power system was poised for a predominant place as a preferred system for use, with the availability of batterie’s for storage, d.c. generators for conversion of mechanical energy into electrical form and d.c. motors for getting mechanical outputs from electrical energy.

1. Basic Principles Electric machines can be broadly classified into electrostatic machines and electro- magnetic machines. The electrostatic principles do not yield practical machines for commer- cial electric power generation. The present day machines are based on the electro-magnetic principles. Though one sees a variety of electrical machines in the market, the basic under- lying principles of all these are the same. To understand, design and use these machines the following laws must be studied. 1. Electric circuit laws - Kirchoff′s Laws 2. Magnetic circuit law - Ampere′s Law 3. Law of electromagnetic induction - Faraday′s Law 4. Law of electromagnetic interaction -BiotSavart′s Law Most of the present day machines have one or two electric circuits linking a common magnetic circuit. In subsequent discussions the knowledge of electric and magnetic circuit laws is assumed. The attention is focused on the Faraday’s law and Biot Savart’s law in the present study of the electrical machines.

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 9, September 2014, Pg. 528-531

2. EMF Equation Consider a D.C generator whose field coil is excited to produce a flux density distribution along the air gap and the armature is driven by a prime mover at constant speed. Let us assume a p polar d.c generator is driven at n rps. The excitation of the stator field is such that it produces a φ Wb flux per pole. Also let z be the total number of armature conductors and a be the number of parallel paths in the armature circuit. In general, as discussed in the earlier section the magnitude of the voltage from one conductor to another is likely to very since flux density distribution is trapezoidal in nature. Therefore, the total average voltage across the brushes is calculated on the basis of average flux density Bav. If D and L are the rotor diameter and the length of the machine in meters then area under each pole is . Hence average flux density in the gap is given by

We thus see that across the armature a voltage will be generated so long there exists some flux per pole and the machine runs with some speed. Therefore irrespective of the fact that the machine is operating as generator or as motor, armature has an induced voltage in it governed essentially by the above derived equation. This emf is called back emf for motor operation. Ritika Gautam,IJRIT

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 9, September 2014, Pg. 528-531

3. No Load Operation When a d.c machine operates absolutely under no load condition, armature current is 0. Under such a condition T

e

developed is zero and runs at constant no load speed. In the absence of any I , the flux per pole φ, inside the machine a

is solely decided by the field current and lines of force are uniformly distributed under a pole

4. Loaded operation A generator gets loaded when a resistance across the armature is connected and power is delivered to the resistance, The direction of the current in the conductor is decided by the fact that direction of T will be opposite to the e

direction of the rotation. It is therefore obvious to see that flux per pole φ, developed in the generator should be decided not only by the mmf of the field winding alone but also by the armature mmf as well as the armature is carrying current now. By superposing the no load field lines and the armature field lines one can get the resultant field lines pattern .The tip of the pole which is seen by a moving conductor first during the course of rotation is called the leading pole tip and the tip of the pole which is seen later is called the trailing pole tip. In case of generator mode we see that the lines of forces are concentrated near the trailing edge thereby producing torque in the opposite direction of rotation. How the trapezoidal no load field gets distorted along the air gap of the generator . In this figure note that the armature mmf distribution is triangular in nature and the flux density distribution due to armature current is obtained by dividing armature mmf with the reluctance of the air gap. The reluctance is constant and small at any point under the pole. This means that the armature flux density will simply follow the armature mmf pattern. However, the reluctance in the q-axis region is quite large giving rise to small resultant flux of polarity same as the main pole behind in the q-axis. In the same way one can explain the effect of loading a d.c motor Point to be noted here is that the lines of forces gets concentrated near the leading pole tip and rarefied near the trailing pole thereby producing torque along the direction of rotation. Also note the presence of some flux in the q-axis with a polarity same as main pole ahead.

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IJRIT International Journal of Research in Information Technology, Volume 2, Issue 9, September 2014, Pg. 528-531

5. Limitations of this motor model There are several assumptions that we have made in the development of this model, which are very important to keep in mind when using the above relations to implement a dc permanent magnet motor: 1. The torque on the motor has all been lumped into one term, which includes the torque provided by the bearing’s of the motor and other frictional losses occuring. The consequence of this assumption means that in general, the noload speed will only approximately be given by the relation w=V/K, since in practice a small, non-zero frictional load is always present. 2. We have assumed that the inductance of the motor is 0, which is normally good assumption (especially for steady state), but must be re-evaluated if transient response of the motor is to be analyzed and recorded. 3. We have assumed that the resistance of the motor is constant, which is ideally not true but , The resistance of the motor actually changes slightly with speed, and also with the temperature. These changes are normally small, however, so this is a valid assumption unless the motor is operating at very high temperatures. 4. As assumed in the development that we actually know the motor constant, K, and the resistance, R, which is in general NOT the case. The motor specification’s section is devoted to characterizing motors given information which we in practice are able to obtain.

References 1) Shigley, J.E, and Mischke, C.R., Mechanical Engineering Design, 5th Ed., McGraw-Hill, New York 1989. 2) Buchsbaum, Frank, Design and Application of Small Standardized components Data Book 757 Vol. 2, Stock Drive Products, 1983 3) Herman, Stephen. Industrial Motor Control. 6th ed. Delmar, Cengage Learning, 2010. 4. Ohio Electric Motors. DC Series Motors: High Starting Torque but No Load Operation Ill-Advised. Ohio Electric Motors, 2011. Archived 20 July 2011 at WebCite 5. Laughton M.A. and Warne D.F., Editors. Electrical engineer's reference book. 16th ed. Newnes, 2003. 4) 6.William H. Yeadon, Alan W. Yeadon. Handbook of small electric motors. McGraw-Hill Professional, 2001.

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